Wafers of solid material may have surfaces processed by deposition and/or etching. For example, semiconductor devices, electrical conductors, magnetic transducers, radio-frequency circuit elements, lenses, mirrors, and other optical components, and other three-dimensional physical structures on a wafer may be shaped by depositing thin films of material on a surface of the wafer and etching the thin films to selectively remove material. During deposition, a beam from an ion source removes material from a target placed in the beam's path. Some of the material removed from the target by the beam deposits on the wafer to form structures on the wafer's surface. Material may deposit more rapidly on sides of a structure facing the beam than on sides of a structure in the beam's shadow. During etching, a beam of energetic particles emitted from an ion source is incident upon the wafer, leading to removal of material from those parts of the wafer exposed to the beam. Masks may be applied to the side of the wafer being processed to control rates of deposition or etching.
For some wafer processing operations it is important that thin films be deposited and/or etched to a condition of uniform thickness on all sides of structures formed on the wafer. The sides of structures being formed on the wafer may be referred to as “facets”. Facets may face in almost any direction relative to the propagation direction of the incident beam. A facet may be evenly exposed to a beam arriving from some directions but shadowed from the beam from other directions. Rotating a wafer during deposition and etching operations has been used to improve uniformity in rates of deposition or etching for all exposed facets. Tilting a wafer at an angle to an incident beam has also been used to improve uniformity in features formed from thin films. Some systems have further combined linear scan of a wafer with wafer rotation and wafer tilt. A linear scan is performed by carrying a wafer back and forth along a linear track, possibly while the wafer is being rotated at a selected tilt angle. However, in previously known systems with linear scan features, the angle of the linear track is held at a fixed angle relative to the walls of the vacuum chamber while the tilted, rotating wafer traverses back and forth along the track.
Uniform rates of etching and deposition contribute to uniform thin film thickness on facets facing any direction at any position on the face of the wafer. Wafer rotation, wafer rotation with tilt, and wafer rotation with tilt and linear scan may improve uniformity of thin film structures in some directions relative to an incident beam but direction-dependent and position-dependent variations in film thickness still occur. For example, even when all three of these wafer motions are combined, shadowed areas will still be found near thin film structures projecting upward from the wafer surface, and the shadowed areas will have different shadow lengths and thin film thickness for features facing one direction compared to another direction and for features near the center of a wafer compared to features near an wafer edge. Shadow asymmetry and thickness variations occur in part because the beam emitted from an ion source spreads out, or diverges, at an angle to the beam's propagation direction. Beam divergence causes local variations in deposition and etching rates at the surface of the wafer. A beam may have radial variations in intensity and possibly other sources of variation that contribute to variations in rates of deposition and etching at different locations on a wafer. Variations in deposition and/or etching from beam divergence may be radially dependent, that is, variations in thin film deposition thickness or etching depth may be greater near the outer peripheral edge of a wafer than near the center of rotation of the wafer. Collimating the beam to reduce the beam divergence angle may reduce such variations, but collimation is difficult and expensive to implement with the accuracy needed to fabricate sub-micron-sized structures on wafers that larger than about 150 millimeters in diameter.
Thin film thickness variations caused by beam divergence may be reduced by forming a beam with a diameter greater than a diameter of the wafer being processed. The size, weight, control system complexity, operating cost, and expense of the ion source needed to produce such a beam increases with an increase in wafer diameter. The larger the ion source, the more difficult it is to produce a beam with uniform beam density distribution, uniform local beam divergence, and energy distribution across the width of the beam, that is, in a direction normal to the beam propagation direction. For example, a large-diameter beam may be formed from a group of smaller “beamlets” propagating in a common direction. Individual beamlets may have variations in beam density distribution, beam divergence, and energy compared to one another. Such variations across the width of the beam contribute to variation in rates of thin film deposition or etching.